EXPRESSION OF THE STEROIDOGENIC ACUTE REGULATORY PROTEIN IN THE CORPUS-LUTEUM OF THE RABBIT - DEPENDENCE UPON THE LUTEOTROPIC HORMONE, ESTRADIOL-17-BETA

The recent characterization of the mitochondrial protein, Steroidogeni c Acute Regulatory (StAR) protein, as a rate-limiting protein in stero idogenesis prompted us to investigate whether StAR is expressed in the rabbit corpus luteum and whether the expression of StAR is responsive to estradiol-17 beta, the luteotropic hormone in the rabbit. In rabbi ts treated continuously with exogenous estradiol through Day 13 of pse udopregnancy (n = 9), immunoblot analysis revealed that luteal express ion of StAR was stable, ranging from 8.5 to 9.7 U of corrected integra ted optical density. Plasma progesterone concentration (mean +/- SEM) remained elevated in these rabbits (14.3 +/- 2.1 ng/ml). In contrast, expression of StAR decreased in corpora lutea of rabbits deprived of e stradiol for the last 48 and 72 h of the experiment (4.9 +/- 2.2 and 0 .3 +/- 0.2 U, respectively, n = 3 per group), and was associated with a decline in plasma progesterone (0.8 +/- 0.1 and 0.5 +/- 0.3 ng/ml, r espectively). Replacement of estradiol after 48 h of estradiol depriva tion (n = 3) stimulated the reappearance of StAR (10.3 +/- 2.6 U) and the restoration of plasma progesterone (10.4 +/- 4.9 ng/ml). [S-35]Met hionine labeling of proteins in rabbit corpora lutea revealed that sev eral isoforms of StAR protein were specifically synthesized in respons e to estradiol treatment. Collectively, these observations are consist ent with a proposed role for StAR in the mediation of the luteotropic effect of estrogen to promote the synthesis of progesterone in the rab bit.